This invention relates to insulated-gate-field-effect-transistors (IGFETS) and/or metal-oxide-semiconductor field-effect-transistors (MOSFETS) and bipolar transistors and the merger of the two technologies to fabricate both on a single substrate. More specifically, the present invention involves the merger of IG FET's with bipolar transistors by a process in which the technical advantages of each technology complements the other.
In an IGFET, drain and source regions are formed on either side of a channel region. The channel region underlies a gate electrode which is insulated from the channel by an insulating layer, such as SiO.sub.2. The MOS is a specie of IGFET wherein the gate electrode is formed of metal.
One of the major advantages of combining bipolar and MOS and IGFET technology is that the strengths of bipolar and IGFET technologies can be integrated into a single chip to obtain a higher performance and a wider range of functions.
IGFET Digital Circuits have high packing density and low standby power which are important features for large systems. Because IGFET transistors are coupled capacitively, the input resistance of MOS transistors is infinite. There is neither resistive loading nor input current. The latter point is very useful for biasing considerations. Using this feature of infinite input resistance, input stages of analog subsystems can be made to have infinite input resistance. In addition, IGFET technologies, typically, have high precision IGFET capacitors with a small voltage coefficient. Finally, IGFET transistors can be used as near ideal switches by operating them between cutoff and linear regions of operation. Any analog system requiring switches and precision capacitors will benefit from having IGFET structures.
Bipolar transistors, on the other hand, have higher transconductance per collector current and per area. For analog circuits, generally, a DC bias current is flowing at all times. For a given supply voltage, the high transconductance per current translates into a higher transconductance per power. This, in turn, leads to the possibility of achieving a given performance at a lower power consumption with a smaller area using bipolar transistors. Bipolar circuits typically have higher frequency response, especially if the circuits are connected to any kinds of capacitive loads. In addition, bipolar transistors have higher intrinsic gain (arithmetic product of the transconductance and the output resistance). An intrinsic gain of a couple of thousands is easy to achieve using bipolar transistors, while it is difficult to achieve a couple of hundreds using IGFET transistors with a reasonable frequency response. Utilizing the higher transconductance per power and area, and the higher frequency response, a better driver can be made with bipolar transistors. Bipolar circuits also have a better matching and a smaller 1/f noise, which are crucial requirements for high precision circuits. For differential pairs of transistors, the offset voltage resulting from the mismatch of the transistor pair is inversely proportional to the ratio between transconductance and current. Because the bipolar transistors have a higher transconductance per current, the off-set voltage for bipolar differential pairs is lower than that of the IGFET pairs. In addition to this, IGFET transistors have additional mismatch due to the variations of the threshold voltage. This lower off-set voltage for bipolar differential pairs has already been exploited in the sense amplifiers of BICMOS Static RAMs to lower the voltage swing and the access time (Watanabe et al., "High Speed BICMOS VLSI Technology with Buried Twin Well Structure", IEDM Tech. Digest, p. 423, 1985).